Brake pedal sinks slowly

Bill47 · 29-08-2007, 16:30

Hi, a few days ago I replaced the front brake pads on my Octavia. Once reassembled I had very soft brakes and the pedal slowly sank under pressure. The car's done 191k miles. A mechanic friend said the the pad replacement procedure had probably forced fluid back past the master cylinder seals. I therefore put a new master cyliner on today. The brakes are still soft (OK new pads take a while to bed in) but with the engine on the brake pedal still sinks slowly. It doesn't sink with the engine off.

We've put new fluid in and bled the whole system very carefully and there are no leaks anywhere. On the road I can do an emergency stop so that the ABS kicks in. Could the sinking pedal be caused by the new master cylinder being defective or should I be looking for something else?

One other thing - while it's sinking it seems to go past a point in it travel where a distinct click can he heard and felt through the pedal.

Thanks, William

mattbeef · 29-08-2007, 16:44

Its not the slave cylinder is it?
Not much help im afraid as thats a random guess

Bill47 · 29-08-2007, 16:54

Slave cylinders seem dry so I'd guess not. Thanks for the suggestion.

I read somewhere that the ABS pump might also need to be bled. Would air in this pump cause that 'sinking feeling'?

Lummox · 29-08-2007, 17:29

Did you have bleed nippled on the new master cylinder?

If you pump the brake pedal does it go hard?

How many miles have you done on the pads?

Lummox · 29-08-2007, 17:31

If it doesnt sink while the engine is off then it cant be air in the system or a faulty master cylinder as it would do it all the time.

I'd clamp off the front brake lines at the flexi and see if the brake pedal still sinks.

Bill47 · 29-08-2007, 17:41

Hi Lummox, yes the new master cylinder soes have bleed nipples - they were duly bled. The new pads have about 30 miles on them now. With the engine on, if you pump the pedal it does get a bit harder but not much. Also if I put my fut on the pedal with the engine off it's good and hard. With my foot still on the pedal I turn the engone one and the pedal sinks slowly straight away.

Why were you suggesting I clamp off the front brake lines?

simonsheil · 29-08-2007, 19:10

Quote:

Originally Posted by Bill47

Why were you suggesting I clamp off the front brake lines?

To determine if the fault is at the calipers or before this point.

fordfan · 29-08-2007, 19:41

Air in the ABS valve block?, will need bleeding with VAG Com or dealer tool.

29-08-2007, 16:30	#1
Bill47 Briskodian Join Date: Mar 2007 Location: Worcestershire Posts: 15 Members Car: 2001 Octavia TDI Ambiente Thanks: 8 Thanked 0 Times in 0 Posts	Brake pedal sinks slowly Hi, a few days ago I replaced the front brake pads on my Octavia. Once reassembled I had very soft brakes and the pedal slowly sank under pressure. The car's done 191k miles. A mechanic friend said the the pad replacement procedure had probably forced fluid back past the master cylinder seals. I therefore put a new master cyliner on today. The brakes are still soft (OK new pads take a while to bed in) but with the engine on the brake pedal still sinks slowly. It doesn't sink with the engine off. We've put new fluid in and bled the whole system very carefully and there are no leaks anywhere. On the road I can do an emergency stop so that the ABS kicks in. Could the sinking pedal be caused by the new master cylinder being defective or should I be looking for something else? One other thing - while it's sinking it seems to go past a point in it travel where a distinct click can he heard and felt through the pedal. Thanks, William

29-08-2007, 16:44	#2
mattbeef Briskodian Join Date: Apr 2006 Location: Sheffield Posts: 3,362 Members Car: Red Octy vRS Thanks: 36 Thanked 116 Times in 115 Posts	Re: Brake pedal sinks slowly Its not the slave cylinder is it? Not much help im afraid as thats a random guess __________________ I few bits here and there but im happy to Performance Torque Go on click me you know you want to

29-08-2007, 16:54	#3
Bill47 Briskodian Join Date: Mar 2007 Location: Worcestershire Posts: 15 Members Car: 2001 Octavia TDI Ambiente Thanks: 8 Thanked 0 Times in 0 Posts	Re: Brake pedal sinks slowly Slave cylinders seem dry so I'd guess not. Thanks for the suggestion. I read somewhere that the ABS pump might also need to be bled. Would air in this pump cause that 'sinking feeling'?

29-08-2007, 17:29	#4
Lummox Cupra Bush King Join Date: Sep 2003 Location: Newport Pagnell Posts: 22,458 Members Car: Mk2 Golf GTi 16v and building a 20VT MK2. Images: 2 Thanks: 22 Thanked 753 Times in 633 Posts	Re: Brake pedal sinks slowly Did you have bleed nippled on the new master cylinder? If you pump the brake pedal does it go hard? How many miles have you done on the pads? __________________ www.unit18.co.uk Remapping Service now available, PM for details.

29-08-2007, 17:31	#5
Lummox Cupra Bush King Join Date: Sep 2003 Location: Newport Pagnell Posts: 22,458 Members Car: Mk2 Golf GTi 16v and building a 20VT MK2. Images: 2 Thanks: 22 Thanked 753 Times in 633 Posts	Re: Brake pedal sinks slowly If it doesnt sink while the engine is off then it cant be air in the system or a faulty master cylinder as it would do it all the time. I'd clamp off the front brake lines at the flexi and see if the brake pedal still sinks. __________________ www.unit18.co.uk Remapping Service now available, PM for details.

29-08-2007, 17:41	#6
Bill47 Briskodian Join Date: Mar 2007 Location: Worcestershire Posts: 15 Members Car: 2001 Octavia TDI Ambiente Thanks: 8 Thanked 0 Times in 0 Posts	Re: Brake pedal sinks slowly Hi Lummox, yes the new master cylinder soes have bleed nipples - they were duly bled. The new pads have about 30 miles on them now. With the engine on, if you pump the pedal it does get a bit harder but not much. Also if I put my fut on the pedal with the engine off it's good and hard. With my foot still on the pedal I turn the engone one and the pedal sinks slowly straight away. Why were you suggesting I clamp off the front brake lines?

29-08-2007, 19:41	#8
fordfan ECUless Join Date: Mar 2005 Posts: 2,057 Members Car: Thanks: 31 Thanked 246 Times in 215 Posts	Re: Brake pedal sinks slowly Air in the ABS valve block?, will need bleeding with VAG Com or dealer tool.

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